The subject invention relates to digital modulation and is an improvement upon the invention of Scott et al U.S. Pat. No. 3,988,540. According to the Scott et al patent, samples of a data modulated carrier are stored as digital numbers in a memory device. Input data signals are coded into proper address signals for addressing this memory during each modulation period to produce a modulated output signal. Particularly, the address of Scott et al for amplitude and phase modulation is in (r,.theta.) (polar coordinate)form. As taught by Scott et al, two identical memories (ROMS) are used for a four phase-two amplitude scheme.
The Scott et al technique has proven in practice to be well-adapted to generation of various phase and phase-amplitude modulated signal structures. However, extension of the Scott et al approach to a 64 point signal structure, a very large constellation in the state of the art, requires storage of such a large number of quantities to such a degree of accuracy as would tax current VLSI and ROM storage technology. As a practical matter, the complexity involved renders it undesirable to extend the Scott et al structure to a 64 point structure.